Geomorphological response of the salt-marshes in the Tagus estuary to sea level rise

Abstract

Salt-marshes are highly valuable ecosystems due to their role in supporting the aquatic and bird life. Furthermore, many anthropogenic activities such as agriculture, salt production or aquaculture targets these areas. They also act as protective barriers to shores, given their ability to dissipate most of the wave and current energy in high tide. Sea level rise can place these intertidal zones at risk, reinforcing the need to understand their morphosedimentary and dynamic response to the variations on the forcing factors, thus allowing for a better management of these environments. Large expansions of salt-marshes and tidal flats are among the morphodynamic contents of the Tagus estuary. This study focuses on the recent past evolution of Tagus marsh areas in order to understand their geomorphological response to higher sea level scenarios. Cores were taken in four contrasting high salt-marsh expansions in estuarine margins (Trancão – TR, Mouchão da Póvoa – MP, Pancas – PA and Corroios - CO). Marsh surfaces were surveyed using DGPS-RTK and tidal regime characterized at each location. The cores reached at least 1.20m in depth and were subsampled every cm for 210Pb and 137Cs radioisotope determination, allowing the derivation of sedimentation rates. In all locations, accretion rates clearly exceed the post-1920 mean rate of sea level rise (+0.21cm/year, Cascais gauge). Their linear extrapolation into the future, until reaching the upper threshold of marsh surface aggradation (MSHT), suggests that Tagus marginal marshes will not drown under the projected sea level elevation scenarios for the end of the 21th century.